1. Field of the Invention
This invention relates to electrostatic-spray methods and apparatus, and in particular to methods of and apparatus for adding electric charges onto liquid to improve the atomization of the liquid and the transfer efficiency, also called the delivery efficiency, of the liquid particles onto target objects.
2. Related Art
The electrostatic charging of aerosol particles, e.g., solid particulate or liquid droplets, is a commonly practiced method of improving the transfer efficiency of a spraying process, so that the fraction of the sprayed material that reaches and coats the target is maximal, and the fraction that misses the intended target object or target surface region is minimal.
It is well known in the art that when aerosol particles, i.e., solid particles or liquid droplets, are electrically charged with electrostatic charges and sprayed toward a grounded and electrically conducting object, the electrostatic charges on the particles make an electric field that acts as a mutually repulsive force on the particles that tends to move the particles apart from one another. The charges on individual particles act to maintain the particle's size. The collection of charges on the ensemble of particles induces a distribution of charges on the target object, said induced distribution are called the image charges and have the opposite polarity to the particle charges. The image charges make an electric field that attracts the particles toward the target object. This attractive electrical image force can be sufficiently strong so that it is larger than the drag force of the air that acts on the particles. In this manner, the electric field acts to attract the particles onto the target surface and to reduce or overcome the tendency of the particles to stop prior to reaching the target or to be influenced sufficiently by air currents or forces acting in the transverse direction so that the particles do not reach the target surface. In this way, the electric forces act to improve the transfer efficiency and to obtain better coating, i.e., coverage. This can be especially beneficial on curved or hidden surfaces, i.e., surfaces that are not in the direct ‘line of sight’ of the sprayer. Furthermore, if the electrostatic charge in a particle exceeds Rayleigh's Limit (see A. G. Bailey, ch. 3), the particle will break into smaller ones as the repulsive force of the electric charge is strong enough that the surface tension or tensile strength of the particle can no longer hold the liquid droplet or solid particle together.
There are many methods to add electrostatic charge onto particles. Tribo-electric charging is a process whereby the electrons on one material are transferred into or onto the other by friction or by different electronic potentials. Although tribo-electric charging is simple, its charge density is low and the process may be unstable. Corona charging is a process wherein electrons are emitted by field-enhanced emission, usually at the sharp tip or edge of a metallic electrode at high electrical potential, e.g., typically, several 10's of kilovolts, and the electrons are accelerated in the high electric field, make collisions with the air molecules, and cause ionization of the air so that an electrical discharge occurs. Subsequently, electrically charged atoms and molecules, i.e., ions, are produced that make collisions with and electrically charge the aerosol particles. Corona is widely applied in solvent-based spray painting industry (U.S. Pat. No. 6,053,437 and U.S. Pat. No. 5,947,377) because the process can generate high charging current, typically as much as 200 μA, and large improvements in the transfer efficiency are obtained. However, in order to prevent the charging current from leaking to ground potential through the liquid path, especially when the liquid is water-based with low electrical resistivity, the reservoir of the liquid must be isolated with heavy insulation material to maintain the contained liquid at a high potential, i.e., a high voltage. The electrical energy stored in such a high-voltage reservoir is very high and could cause deadly electric shock if the operator is not carefully isolated, i.e., insulated from the high voltage. Typically, such insulation comprises an undesirable contribution to the weight and size of the sprayer unit. Another method, called pre-charge, stores electric charge in the liquid stored in an isolated reservoir. Similar to corona, the pre-charge method could add high electric charge into the liquid and aerosol, but the risk of electric shock is also great. Induction is a process where electrical charge is induced onto the liquid droplets or the solid particles as they separate, e.g., as a liquid jet disintegrates into aerosol droplets, from a grounded nozzle and move in an applied electric field that results from the potential applied to an adjacent electrode. Compared to the corona method of charging, the induction method uses a lower applied high voltage, which is typically in the range of one to a few kilovolts. U.S. Pat. No. 5,704,554 taught a method to embed an electrode inside a spray nozzle, where the liquid is atomized by high-velocity compressed air, and to greatly reduce or prevent electric current from leaking to the grounded nozzle by a sophisticated design. U.S. Pat. No. 6,227,466B1, U.S. Pat. No. 6,138,922 and U.S. Pat. No. 6,053,437 proposed methods to simplify the electric wiring and to share one high-voltage power supply for multiple spray nozzles.
One common problem of all of the above corona and induction electrostatic charging methods is that they require high-speed compressed air to atomize the liquid into fine particles. In U.S. Pat. No. 5,704,554, the liquid is pushed out of the reservoir and broken into particles by the pressure differential that results from the vacuum and the shearing forces created by the compressed air flowing through the nozzle. By having compressed air flowing between the electrode and the liquid, a conduction path between the high-voltage electrode and the grounded liquid can be prevented or at least made a very high impedance so as to avoid current leakage that would significantly reduce the charging voltage on the electrode or comprise a significant power loss. U.S. Pat. No. 6,227,466B1, U.S. Pat. No. 6,138,922 and U.S. Pat. No. 6,053,437 adopted similar methods, which vary in the manner of how the high-voltage and ground potential are connected or conducted to the nozzle area. Although a high-speed compressed-air flow can both effectively break the liquid into fine particles and also prevent the formation of an electrical conduction leakage path between the electrode and the nozzle, the air flow could significantly reduce the transfer efficiency as many liquid particles may be carried away by the high-speed air flow and be deflected from the target surface. In some applications, such a high speed air flow is not desirable because the air flow may dislodge particulate or other contamination from the target surface and spoil the purpose for which the sprayed material is applied. An example is the application of a decontaminant spray. In this case, a high-speed air flow may dislodge and blow contamination, e.g., a chemical or biological agent, from the target surface into the atmosphere or onto an adjacent surface, thus comprising the unwanted spread of the contamination material. Another major problem of using compressed air or gas is that it requires either a source of compressed gas such as a chemical reaction, or a container of compressed gas such as a compressed air cylinder or tank, or a significant expenditure of power to obtain the high air pressure and flowrate that are sufficient for the atomization and aerosol delivery. For field applications, i.e., for a portable sprayer, a typical means for obtaining compressed air is an air compressor with a heavy tank and a powerful motor. In a portable situation, such a compressor must be powered by a huge and heavy battery or a powerful generator, if power receptacles are not available.
Another major limitation of the prior art is that the implementation usually requires a specially designed spray gun and unique nozzles that are much more expensive than regular non-electrostatic spray guns. In fact, the additional high capital cost is why electrostatic spraying is applied only in very small percentage of agricultural and industrial applications. Examples are in agriculture for high price crops and in industry for high price products. Without electrostatics, a significant portion of the spray is usually wasted, e.g., spray that misses the target is called overspray. Examples are found in the spraying of pesticides and paint, where overspray not only makes the cost of the application higher, but it also contributes to causing more pollution. More widespread use of electrostatic spraying can be realized if the cost of the electrostatic-spray equipment is less expensive.
Yet another reason for the limited use of electrostatic spraying is the potential hazard posed by the use of high voltage. In one approach, the spray gun is at high potential, typically 60 kilovolts to 120 kilovolts, and the target is electrically grounded. In this case, the applied electric field between the spray gun and the target acts to attract the particles to the target. However, this approach results in exposed high voltage components and the possibility of the spray acting as a conduction path that could result in an inadvertent contact of personnel with the high voltage, and so means to exclude personnel from the vicinity of the spray gun and spray are necessary. In a more common approach, the spray gun is operated at a lower high voltage, typically one to a few kilovolts. In this case, it is still necessary to ensure that personnel do not come into contact with the high voltage parts so that the use of the sprayer is safe. However, in this case, the applied potential is used principally to obtain the aerosol charging and it is a combination of the initial momentum of the spray and the subsequent image force that transports the particles. To make the use of such electrostatic spraying safe as well as practical and economical, it is necessary that the implementation of the charging method have a configuration that avoids the inadvertent contact and shock of personnel and sensitive equipment.